by Jim Michaels, USA TODAY

by Jim Michaels, USA TODAY

WASHINGTON - The nation's top military leader said the number of cyber intrusions probing the nation's critical infrastructure has increased "seventeenfold" during the past two years.

Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said the Pentagon is investing heavily in capabilities to defend the nation against cyber threats.

Dempsey said the Pentagon is adding 4,000 cyber operators over the next four years and investing $23 billion in cyber security.

The Pentagon is also refining its rules of engagement to clarify command authority for the use of cyber warfare to defend the nation's networks.

"Keep calm and carry on is not appropriate to the threat," Dempsey told an audience at the Brookings Institution think tank.

The world's growing dependency on computers, which are increasingly networked together, has made the nation's infrastructure, such as power grids and water systems, vulnerable to attack.

Dempsey said the trend also gives the U.S. military a new "asymmetric advantage" in future wars. The Pentagon has said it is developing capabilities, such as the ability to use cyber warfare to blind an enemy radar.

Dempsey did not mention specific capabilities but said, "cyber is strengthening the Air Force's ability to achieve global reach."

The immediate worry for military planners, however, is the growing number of small scale attacks that occur daily on U.S. companies.

Washington has accused China and other nations of frequent network intrusions designed to steal proprietary information to give its companies an edge.

Most such attacks fall short of an act of war, U.S. officials have said. But defining when such attacks rise to such a level is a question for political leaders, Dempsey said.

What is clear is that some nations have developed the capability to launch massive cyber attacks that could cripple an adversary.

"I do think there are capabilities out there that are so destructive in nature and potential that it would be very difficult not to see them as acts of war," Dempsey said.